In one of the commercially used methods for manufacturing titanium dioxide pigment particles, known as the chloride process, titanium tetrachloride (TiCl4) is reacted with an oxidizing gas, such as oxygen, air, etc., and with certain additives in a tubular reactor to form titanium dioxide and chlorine gas:TiCl4+O2→TiO2+2 Cl2 
The TiO2 particles are subsequently separated from the chlorine gas. Known additives are AlCl3 as a rutilizing agent and steam or alkali salts as a nucleating agent.
The oxidation process is customarily performed in one stage, i.e. the reaction components (educts), oxygen and gaseous titanium tetrachloride, are each added at only one inlet point of the reactor. Owing to the high activation energy of TiCl4 oxidation, the educts must, before addition, be heated to such a degree that an adiabatic mixed temperature of at least approx. 740° C. is reached. The oxidation reaction is highly exothermal, meaning that an adiabatic reaction temperature of approx. 1,850° C. is reached following complete conversion. Before the pigment produced is separated from the gas mixture in a filter, this mixture has to be cooled to a maximum of 450° C. in order to avoid damage to the filter. This process is energetically unsatisfactory because large amounts of heat are dissipated into the cooling-water system. Single-stage oxidation is also disadvantageous in terms of product quality, since the extensive hot zone for oxidation promotes the formation of hard TiO2 aggregates.
Consequently, there are various developments in the prior art for operating the process on a multistage basis. According to GB 1 064 569, both TiCl4 and O2 are added in two stages, in which context the respective quantity of O2 is sufficient for completely oxidizing the respective quantity of TiCl4.
The teaching according to U.S. Pat. No. 4,053,577 provides for a maximum of one of the educts to be introduced into the reactor in two stages.
In the methods according to GB 2 037 266, U.S. Pat. No. 4,803,056 and EP 0 583 063 B1, gaseous TiCl4 is introduced into a hot oxygen stream in two or more stages.
The method according to EP 0 852 568 B1 provides for not only the TiCl4 to be added in two stages, but also the oxygen. However, the object of this method is effective control of the mean TiO2 particle size, and thus of the tone of the TiO2 pigment base material. In this case, TiCl4 vapour having a temperature of about 400° C. is first fed into an oxygen stream with a temperature of about 950° C. The TiO2 particles are formed, and particle growth takes place, in the downstream reaction zone. TiCl4 vapour heated to a lesser extent (approx. 180° C.) is added at a second inlet point. Oxygen having a temperature between 25° C. and 1,040° C. is introduced at the second inlet point, the temperature of the mixture being sufficient to initiate the reaction.
The multi-stage method according to U.S. Pat. No. 6,387,347 is additionally said to reduce agglomeration. To this end, the previously heated, gaseous TiCl4 stream is split into two part streams before addition to the reactor. One part stream is oxidized in the first stage of the reactor. The second part stream is cooled by injection of liquid TiCl4 (de-superheating) and then added to the reactor. De-superheating takes place outside the reactor, the temperature not falling below the condensation temperature of the overall stream.
US 2007/0172414 A1 discloses a multistage method for the reaction of TiCl4 and O2 in which gaseous TiCl4 is fed into the reactor in the first stage, and liquid TiCl4 in the second stage and wherein the oxygen is present in a hyper-stoichiometric amount in the first stage. This method permits energy savings and improvement of the particle size range.